1. Field of the Invention
This invention relates generally to Josephson junctions and, more particularly, to injection of fluxons into annular Josephson junctions.
2. Discussion of Related Art
Long Josephson junctions are interesting systems from the perspective of providing a workbench for fundamental investigations of a variety of superconducting phenomena, as well as having various applications in cryoelectronics. For an elementary introduction see SUPERCONDUCTIVITY by Charles P. Poole, Jr., Horacio A. Farach and Richard J. Creswick (Academic Press, 1995), pp. 442-444 and references cited. Long Josephson junctions are also useful for studying basic properties of solitary waves (solitons). Solitons of the simplest type are topological kinks and are able to propagate. A well-known example of such a soliton is the elementary quantum of magnetic flux "PHgr"0 (also called a fluxon, or Josephson vortex) in a long Josephson junction. See, for example, A. Barone and G. Paternò, PHYSICS AND APPLICATIONS OF THE JOSEPHSON EFFECT (Wiley, N.Y. 1982); A. V. Ustinov, Physica D 123, 315 (1998). A fluxon in a long Josephson junction can be caused to move along the junction by the application of a bias current IB flowing across the junction. The resulting motion of such a fluxon gives rise to a dc voltage Vdc across the junction, which is proportional to the fluxon""s mean velocity "ugr". Thus, a measurement of Vdc as a function of IB provides a useful way to gain information about properties of the fluxons, including the number of fluxons present.
Our primary concern herein is with long Josephson junctions which, for economy of language, we refer to simply as xe2x80x9cjunctionsxe2x80x9d understanding thereby that long Josephson junctions are understood. Explicit descriptions of junctions having other shapes will be included when necessary for clarity.
An important property of an annular long Josephson junction results from the quantization of magnetic flux in a superconducting ring. The annular junction is a topologically closed system such that the number of initially trapped fluxons is conserved and new fluxons can be created only in the form of fluxon-antifluxon pairs. See, e.g., A. Davidson, B. Dueholm, B. Kryger, and N. F. Pedersen, Phys. Rev. Lett. 55 2059 (1985). Fluxon motion in annular junctions occurs under periodic boundary conditions and without any reflections from boundaries, thereby avoiding many mathematical and physical complications that occur for fluxon motion in other junction shapes. One source of the interest in investigating annular junctions derives from fundamental aspects of the Berry phase effect that arises in annular junctions. (see e.g. F. Gaitan, Phys. Rev. B 63, 104511-1 (2001); and V. Plerou and F. Gaitan, Phys. Rev. B 63, 104512-1 (2001)). Other sources of interest in annular junctions arise from the phenomena of Cherenkov radiation by solitons that can be studied therein. (see, for example, E. Goldobin, A. Wallraff, N. Thyssen, and A. V. Ustinov, Phys. Rev. B 57, 130 (1998); and A. Wallraff, A. V. Ustinov, V. V. Kuring, J. A. Shereshevsky, and N. K. Vdovicheva, Phys. Rev. Lett. 84, 151 (2000)). Applications with a view towards the development of practical devices can also be investigated with annular junctions.
Ring-shaped annular junctions have also been proposed as microwave sources with high stability and very narrow radiation line width (for example, see U.S. Pat. No. 4,181,902 to A. C. Scott). Annular junctions with trapped fluxons have also been suggested as radiation detectors in which they have an advantage of a stable operation point at a finite voltage. (See for example, C. Nappi and R. Christiano, Appl. Phys. Lett. 70, 1320 (1997); M. P. Lisitskii et al., Nucl. Instr. and Methods in Phys. Research A 444, 476 (2000)). More recently, annular junctions of special shapes have been proposed for the creation, storage and manipulation of quantum bits (xe2x80x9cqubitsxe2x80x9d) in the form of fluxons (see A. Wallraff, Y. Koval, M. Levitchev, M. V. Fistul, and A. V. Ustinov, J. Low Temp. Phys. 118, 543 (2000)); and fluxon ratchets (E. Goldobin, A. Sterk, and D. Koelle, Phys. Rev. E 63, 031111 (2001), and Carapella, Phys Rev. B 63, 054515 (2001)). Fluxons in Josephson transmission lines, which are discrete analogs of long Josephson junctions, have been proposed as on-chip clocks by V. Kaplunenko, V. Borzenets, N. Dubash, and T. Van Duzer, Appl. Phys. Lett. 71, pp 128-130 (1997), Y. Zhang and D. Gupta, Supercond. Sci. Technol., 12, pp 769-772 (1999), D. Gupta and Y. Zhang, App. Phys. Let. 76, pp. 3819-3821 (2000), and U.S. Pat. No. 6,331,805, xe2x80x9cOn-Chip long Josephson Junction (LJJ) Clock Technologyxe2x80x9d, to Gupta et al.
A significant problem in utilizing fluxon states in annular junctions is preparation of the initial state of the system containing a single or a predetermined number of fluxons. For example, in order to realize a state having a single fluxon, a single magnetic flux quantum has to be trapped in the junction, i.e., between its superconducting electrodes. The only reliable and reproducible technique for trapping fluxons in an annular junction that has been previously known and used requires rather exotic and complicated apparatus, namely a low temperature scanning electron (or laser) microscope. See e.g. A. V. Ustinov, T. Doderer, B. Mayer, R. P. Huebener and V. A. Oboznov, Europhys Lett. 19, 63 (1992). Other known methods for trapping magnetic flux in an annular junction can be used while cooling the sample below the critical temperature of its superconducting electrode(s). These other methods are based on either sending a current through an additional specially designed coil placed on top of the annular junction (see I. V. Vernik, V. A. Oboznov and A. V. Ustinov, Phys. Lett.A 168, 319 (1992)), or applying a small bias current directly through the junction (see A. V. Ustinov, Pis""ma Zh. Eksp. Teor. Fiz. 64, 178 (1996) [Sov. Phys. JETP Lett. 64, 191 (1996)]; and I. V. Vernik, S. Keil, N. Thyssen, T. Doderer, A. V. Ustinov, H. Kohlstedt, and R. P. Huebener, J. Appl. Phys. 81, 1335 (1997)). Unfortunately, the latter techniques are not sufficiently reproducible and require heating of a junction to high temperature. Moreover, fluxons trapped in such ways often suffer from parasitic pinning due to Abrikosov vortices which become trapped in superconductive electrodes. Thus, there is a need for a system to inject a single fluxon, or a known number of fluxons, in a controlled manner into an annular Josephson junction. The present invention is directed to providing such a system.
The present invention relates to a fluxon injection system including injection electrodes separated by a distance D in contact with one terminal of an annular Josephson junction. Fluxons are trapped on the annular Josephson junction when an injection current of sufficient magnitude is injected through the injection electrodes.
Application of an injection current causes current to flow from one of the injection electrodes into a superconducting electrode and across the Josephson barrier. The current is collected by another injection electrode on the same superconducting electrode so that the total current across the Josephson barrier remains zero. A magnetic flux is thus created in the region between the injection electrodes. As the magnitude of the magnetic flux created by the injection current increases and becomes larger than the elementary quantum of magnetic flux, "PHgr"0, it may become energetically favorable for a compensating negative flux to be created. If the induced flux exceeds "PHgr"0, the remaining positive flux on the annular Josephson junction can exist in solitary form and become a fluxon. The induced flux on the annular Josephson junction is removed when the injection current is removed. In this case, the compensating negative flux annihilates the remaining solitary positive flux and the junction is then free of fluxons. Thus, control of the properties of the system, including the current flow between the injection electrodes and the electrode spacing, results in controlled insertion of fluxons into the annular junction. Such controlled fluxon initial states on annular Josephson junctions can be used in connection with clock references, radiation detectors, and fluxon oscillators, among other applications. Shaped junctions can advantageously be employed along with the injection systems of the present invention for initialization of qubits for quantum computing, among other applications.